Polar ethylene-based polymer, cycloolefin polymer blends

ABSTRACT

Blends comprising a polar derivative of an ethylene based polymer and a cycloolefin polymer are prepared. The ethylene based polymer preferably comprises at least 60 weight % of ethylene, relative to the weight of the polymer and polar monomers selected from the group of carboxylic acids, alkyl acrylates and vinyl acetates. It is preferably 0.01 to 99.5% neutralized with metal ions selected from sodium, potassium, zinc, calcium, magnesium, lithium, aluminum, nickel, and chromium. The cycloolefin polymer is preferably an ethylene-norbornene copolymer. The invention also relates to the applications of such blends as sealant layers and film packagings. The perforation resistance, the heat seal initiation temperature and the hot tack strength of such films are improved.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/269,899, filed Feb. 20, 2001.

FIELD OF INVENTION

[0002] The present invention relates to blends comprising a polarderivative of an ethylene based polymer and cycloolefin polymers, and totheir applications in sealants for packaging films.

BACKGROUND OF THE INVENTION

[0003] In the packaging industry, it is conventional that a product ispackaged in a plastic film or a rigid plastic package which is then heatsealed. The film forming the package usually comprises a substratelayer, a tie-layer and a sealant layer. The sealant layer is the topmostlayer of the film and is in direct contact with the packaged item. Itmust have a high perforation resistance, especially for packaging of dryfood. The seal must also possess a mechanical resistance sufficient tomaintain the tight-seal properties during storage and transport. It ispreferable that the package be opened without destroying the integrityof the film itself. In other words, the seal should be peelable, that isto say that it should be easily openable by pulling with hands, withoutthe need of scissors or other instruments.

[0004] In addition, in industry, it is desired that these packagingfilms be suitable for high speed machine packaging. In this view, it isuseful to provide a sealant layer that can be sealed at relatively lowtemperatures and that has a very high hot tack in order to allow forhigher packaging line speeds of packaging machines. It would thereforebe useful to have sealant layers made out of polymers showing arelatively low heat seal initiation temperature.

[0005] It is known to mix cyclic olefinic copolymers and classicalpolyolefins in order to manufacture sealant layers for packaging films.Nevertheless, the heat seal initiation temperature of those sealantlayers are still very high. They are usually around 100° C. Moreover,the perforation resistance of these films is also not satisfactory.

[0006] Therefore, there is still a need for sealant layers for packagingfilms that would have a relatively low heat seal initiation temperature,a good perforation resistance and also a good hot tack value.

SUMMARY OF THE INVENTION

[0007] Now, it has been surprisingly found that the combination of twoincompatible specific types of polymers lead to a sealant layer showingexcellent properties as regards stiffness, perforation resistance, heatseal strength and hot tack strength.

[0008] Incompatible polymers, as used herein, means polymers that arenot soluble in each other, in other words, polymers having solubilityparameters which are significantly different from each other, preferablywhich differ from at least 0.5 units. Solubility parameters are definedin J Bandrup, E. H. Immergut (Ed), Polymer Handbook, Page IV 337 ff NewYork, 1975.

[0009] A first aspect of the invention is a blend comprising at least apolar derivative of an ethylene based polymer and at least a cycloolefinpolymer.

[0010] Another aspect of the invention is a film comprising the blendabove.

[0011] Another aspect of the invention is a sealant layer comprising theabove film.

[0012] Another aspect of the invention is a packaging comprising thesealant layer above.

[0013] Another aspect of the invention is a method for improving theperforation resistance of a film made of a polar derivative of anethylene based polymer comprising the step of introducing therein fromabout 0.1 to 50% by weight, relative of the weight of the film, of acycloolefin polymer.

[0014] Another aspect of the invention is a method for reducing the heatseal initiation temperature of a film made of a polar derivative of anethylene based polymer comprising the step of introducing therein fromabout 0.1 to 50% by weight, relative of the weight of the film, of acycloolefin polymer.

[0015] Another aspect of the invention is a method for stabilizing theheat seal strength of a film made of a polar derivative of an ethylenebased polymer over a wide temperature range comprising the step ofintroducing therein from about 0.1 to 50% by weight, relative of theweight of the film, of a cycloolefin polymer.

[0016] Another aspect of the invention is a method for increasing thehot tack strength of a film made of a polar derivative of an ethylenebased polymer range comprising the step of introducing therein fromabout 0.1 to 50% by weight, relative of the weight of the film, of acycloolefin polymer.

[0017] The film of the invention has improved resistance to impact,improved hot tack value. It can also have a significantly reduced heatseal initiation temperature (see Examples).

[0018] Although the ethylene based polymer and the cycloolefin polymerof the invention are incompatible, their combination leads to improvedmechanical properties of the film made of their blend. Moreover, whenopening the seal, the film made of the blends of the invention have thepotential to delaminate and therefore offer a perfectly controlled peeluse as well as high perforation resistance, which is an advantage overcommercial peel systems which are usually poor in perforationresistance.

[0019] The film of the invention shows an excellent perforationresistance. The packagings made of this film are therefore moreresistant. This is very important in food packaging, where bones forexample may damage the film. It also shows a low seal initiationtemperature, which means that packages made of this film can be sealedat lower temperatures than before. This is particularly important inindustrial processes where each operation must be rapid. The film of theinvention also shows high hot tack strength. That means that the sealingoperation, in particular in vertical form fill (VFF) machines and inflow wrapping processes where the goods are packaged under slightoverpressure in modified atmosphere (MAP), can be speeded upsignificantly. One does not have to wait a certain time until thesealing is effective. This also allows more packages to be heat sealedin less time. The film can be wrapped in high speed automatic wrappingmachines, increasing the productivity in the industrial processes.

[0020] The films of the invention can find use in food packagings likecereals, meat, soup packagings.

DETAILED DESCRIPTION OF THE INVENTION

[0021] The first essential component of the composition of the inventionis a polar derivative of an ethylene based polymer. “Polar derivative ofan ethylene based polymer”, as used herein, means an ethylene basedpolymer where part of the hydrogen atoms in the hydrocarbon chain ofpolyethylene is substituted by at least one atom chosen from oxygen ornitrogen. This polar derivative of an ethylene based polymer ispreferably a copolymer or a terpolymer. It preferably comprises at least60 weight % of ethylene, relative to the weight of the polymer, andpolar monomers selected from the group of carboxylic acids, alkylacrylates and vinyl acetates.

[0022] In a preferred embodiment of the invention, the polar derivativeof an ethylene based polymer is of ethylene; α,β-ethylenicallyunsaturated C₃-C₈ carboxylic acid, and optionally one or more softeningcomonomers copolymerizable with ethylene. The α,β-ethylenicallyunsaturated C₃-C₈ carboxylic acid is preferably present in an amount ofabout 5-15 weight %, relative to the weight of the polymer. Thesoftening monomer is preferably present in an amount of less than about25 weight %, relative to the weight of the polymer. Acrylic andmethacrylic acids are preferred acid comonomers. The softening comonomercan be an alkyl acrylate selected from the group consisting of methylacrylate, n-propyl-, iso-butyl-, n-butyl-, n-octyl-, 2-ethylhexyl- and2-methoxyethyl-acrylates. The preferred alkyl acrylates are iso-butyl-,n-butyl-, 2-ethylhexyl-, and 2-methoxyethyl-acrylates. The softeningcomonomer can also be an alkyl vinyl ether selected from the groupconsisting of n-butyl-, n-hexyl-, 2-ethylhexyl-, and2-methoxyethyl-vinyl ether. The preferred alkyl vinyl ethers are n-butylvinyl ether and n-hexyl vinyl ether.

[0023] The polymer is optionally about 0.01 to 99.5% neutralized withmetal ions selected from groups Ia, Ib, IIa, IIIa, IVa, VIb, and VII ofthe Periodic Table of Elements such as sodium, potassium, zinc, calcium,magnesium, lithium, aluminum, nickel, and chromium. Such neutralizedethylene acid copolymers are known in the art as ‘ionomers’. Typically,neutralization will be from about 10-70%. Preferably the copolymer hasfrom about 35 to about 70% of the carboxylic acid groups ionized byneutralization with metal ions selected from the group consisting ofsodium, potassium, zinc, calcium, and magnesium. Ionomers and theirmethods of manufacture are described in U.S. Pat. No. 3,264,272.

[0024] Suitable polymers for use in the present invention are theionomers commercially available under the trademark Surlyn® from E. I.du Pont de Nemours and Company and the ethylene acid copolymersavailable under the trademark Nucrele from E. I. du Pont de Nemours andCompany.

[0025] In another preferred embodiment of the invention, the polarderivative of an ethylene based polymer is of ethylene, alkyl(meth)acrylate and optionally one vinylalkylsilane termonomer or carbonmonoxide. The alkyl (meth)acrylate monomer is preferably present in anamount of about 5-40 weight %, relative to the weight of the polymer.The alkyl group of the alkyl (meth)acrylate monomer is preferably methylbut can be a higher alkyl group up to octyl. The vinylalkylsilanetermonomer is preferably present in an amount of less than 3 weight %,relative to the weight of the polymer. The preferred vinylalkylsilanetermonomers are selected from vinylmethylsilane and vinylethylsilane.The carbon monoxide monomer is preferably present in an amount less than15 weight %. Relative to the weight of the polymer. Such polymers andtheir preparation are described in U.S. Pat. No. 3,780,140. Thesepolymers can be prepared by the various methods known in the art such ashigh pressure free radical polymerization. Suitable polymers for use inthe present invention are the products commercially available from E. I.du Pont de Nemours under the trademarks Elvaloy® AC, Elvaloy® HP.

[0026] Other polar derivatives of an ethylene based polymer suitable forthe present invention are the polymers of ethylene and vinyl acetatecommercially available from E. I. du Pont de Nemours under the trademarkElvax®.

[0027] The polar derivative of the ethylene based polymer is usuallypresent in the blend of the invention in an amount from about 50 weight% to 99.9 weight %, relative to the weight of the blend.

[0028] The second essential component of the blend of the invention is acycloolefin polymer. Suitable cycloolefin polymers for the inventionhave a mean molecular weight Mw (weight average) in the range from 200to 100,000. They are substantially amorphous, i.e. have a crystallinityof less than 5% by weight. They preferably show a glass transitiontemperature Tg, which is generally in the range from 0 to 300° C. Thepolydispersity Mw/Mn of the cycloolefin polymers is preferably from 1 to5.

[0029] Cycloolefin polymers (COPs) are homopolymers built up from onlyone type of cycloolefins or copolymers built up from cycloolefins andcomonomers (COCs), where the comonomer content is at most 20% by weight,based on the weight of the cycloolefin polymer. Cycloolefins are mono orpolyunsaturated polycyclic ring systems, such as cycloalkenes,bicycloalkenes, tricycloalkenes or tetracycloalkenes. The ring systemscan be monosubstituted or polysubstituted. Preference is given to thecycloolefins which are built up from monoalkylated or unsubstitutedcycloolefins. Particularly preferred cycloolefin homopolymers arepolynorbornene, polydimethyloctahydronaphthalene, polycyclopentene andpoly(5-methyl)norbomene. The cycloolefin polymers can also be branched.Products of this type can have comb or star structures.

[0030] If desired, the above-described cycloolefins can also becopolymerized with comonomers. In a preferred embodiment of theinvention, these cycloolefin copolymers (COCs) contain up to 20% byweight, preferably 1-15% by weight, in particular 1-8% by weight, basedon the weight of the COC, of comonomer. Preferred comonomers are olefinshaving 2 to 6 carbon atoms, in particular ethylene and butylene.

[0031] Preferred cycloolefin polymers of the invention areethylene-norbornene copolymers.

[0032] The cycloolefin polymers can be prepared with the aid oftransition-metal catalysts. Preparation processes are described, forexample, in DD-A-109 225, EP-A-0 407 870, EP-A-0 485 893 and U.S. Pat.No. 5,869,586, 6,068,936 and W098/27126 which are incorporated herein byway of reference. Molecular weight regulation during the preparation canadvantageously be effected using hydrogen. Suitable molecular weightscan also be established through targeted selection of the catalyst andreaction conditions. Details in this respect are given in the abovementioned specifications.

[0033] Suitable cycloolefin for the invention are the products soldunder the trademark Topas® by Ticona.

[0034] The cycloolefin polymer is usually present in the blend of theinvention in an amount from about 0.1 weight % to 50%, relative to theweight of the blend.

[0035] The blend of the invention may comprise the usual additivesincluding plasticizers, stabilizers, antioxidants, ultraviolet rayabsorbers, hydrolytic stabilizers, anti-static agents, dyes or pigments,fillers, fire-retardants, lubricants, reinforcing agents such as glassfiber and flakes, processing aids, for example release agents, and/ormixtures thereof.

[0036] The blend of the invention can be prepared by any extrusionprocess, for instance in the following manner: a blend is prepared bysimply mixing the dry components, as a salt and pepper blend. This blendis then compounded in an extruder. The exiting compound is granulated.Alternatively, the blend can also be fed in a film converting extruderwithout compounding.

[0037] The film of the invention can be prepared by extrusion asfollows: the granulates obtained as hereinabove are melted in a suitableextruder and converted into a film with at least one individual layerusing a converting technique. Suitable converting techniques are forinstance blown film extrusion, cast sheet extrusion, extrusion coating.

[0038] The incorporation of about 0.1 to 50% of a cycloolefin polymer ina film made of a polar ethylene based polymer according to the inventioncan have many advantages: it improves the perforation resistance of saidfilm, it reduces its heat seal initiation temperature, it increases itshot tack strength. It can also stabilize its heat seal strength over awide temperature range, for instance for temperatures ranging from 80°C. to 150° C. It then allows to obtain a low seal strength and an easyopenability of the film.

[0039] The packaging of the invention may be prepared according to theclassical methods of preparation of such packagings made from films.

[0040] The invention will be further described in the followingExamples.

EXAMPLES

[0041] Description of Ingredients

[0042] Materials used in the Examples set forth below are as follows,identified by the respective trademarks and trade designations:

[0043] Ionomer 1: copolymer of ethylene with 15% methacrylic acid and amelt flow index (MFI) of 0.7, 58% neutralized with Zn, commerciallyavailable from E. I. du Pont de Nemours and Company.

[0044] Ionomer 2: copolymer of ethylene, with 10% methacrylic acid andMFI of 1.5, 38% neutralized with Zn, commercially available from E. I.du Pont de Nemours and Company.

[0045] Elvaloy® AC 2116 : copolymer of ethylene and 16% ethylacrylate,MFI of 1, commercially available from E. I. du Pont de Nemours andCompany.

[0046] TOPAS® 8007: copolymer of ethylene and norbornene, commerciallysold by TICONA

[0047] TOPAS® 6013: copolymer of ethylene and norbornene, commerciallysold by TICONA

[0048] The Melt Flow Index (MFI) of the above compounds is measuredaccording to ASTM D 1238@190° C./2.16kg.

[0049] The following compositions were prepared. All amounts are givenin parts by weight relative to the total weight of the composition.Composition 1 2 3 4 5 6 7 8 9 lonomer 1 100 90 80 80 lonomer 2 100 80 80Elvaloy ® AC 100 80 2116 Topas ® 8007 10 20 20 20 Topas ® 6013 20 20

[0050] Compositions 1, 5, and 7 are comparative compositions.

[0051] Compositions 2-4, 6 and 8-9 are compositions according to theinvention.

[0052] The compositions are compounded in a 24 mm PRISM twin screwextruder using a high shear screw. All ingredients were prepared as asalt and pepper blend and granulated after exiting the extruder.

[0053] The compounding temperature conditions were as such: Feed Zone 1Zone 2 Zone 3 Die Melt 205 210 230 240 240 240

[0054] Part of the granulate obtained this way is melted in a Brabender24 mm single screw extruder where a blown film with a thickness between70 and 100 micrometer was extruded. The melting temperature of theBrabender single screw extruder was as follows: Feed Zone 1 Zone 2 Zone3 Die Melt 170 195 220 235 235 235

[0055] This film was subsequently subjected to the following tests:

[0056] Static Perforation Resistance

[0057] Static perforation resistance was tested according to the testdescribed in New sealant Concepts for Flexible packaging, Proc SP98Specialty Films 98, 14th Annual World Congress, Duesseldorf 1998,Session IV-2 (ed. MBS, Au bei Zuerich), K. Hausmann, D. Flieger. Thefilm is stretched on a metal plate. It is then subjected to staticweight which is positioned on a 90 degree sharp needle. The needlegradually perforates the film. The time which is required to perforatethe film is recorded.

[0058] For each composition 1 to 6, a film of 100 microns was preparedas described above. Each film was subjected to a static weight of 220 g.For each composition 7 and 8, a film of 70 microns was prepared asdescribed above. Each film was subjected to a static weight of 50 g. Theresults (time in seconds required to perforate the film) are collectedin the following table: Composition 1 2 3 4 5 6 7 8 Time in sec 121 152860 458 8 157 3 1896

[0059] It is clear from these results that the incorporation of acycloolefin polymer in a film made of a polar derivative of an ethylenebased polymer significantly increases its perforation resistance,despite the incompatibility of the two polymers.

[0060] Heat Seal Strength Test

[0061] For each composition 1, 3, 4, 5, 6, 9, films of 100 microns wereprepared as described above. Heat Seal performance has been tested byfirst heat sealing the films to each other on a Kopp heat sealer attemperatures between 80 and 160° C. using a contact time of 0.5 sec anda pressure of 0.5 MPa. After 24 h the seal strength was measured on aZwick tensile testing device at a crosshead speed of 100 mm/min. Theresults are given in N/15 mm, in function of the sealing temperature.

[0062] The seal initiation temperature (SIT) is the temperature wherethe seal strength is>0.5N/1 5mm. The SIT measured for the compositionsare collected in the following table: Composition 1 3 4 5 6 9 SIT in °C. 80-90 <80 <80 80-90 <80 <80

[0063] Heat seal initiation temperature is significantly reduced byadding 20% of a cycloolefin polymer to a polar derivative of an ethylenebased polymer, despite the uncompatibility of the two polymers.Moreover, the cycloolefin polymers Topas® 8007 and Topas® 6013 are knownin the literature to have a melting range between 140-350° C. Theethylene based polymers Ionomer 1 and Ionomer 2 have a meltingtemperature of 85-95° C. It is therefore unexpected and surprising thata 20% addition of a higher melting material to polar derivatives ofethylene based polymers gives a reduction of the seal initiationtemperature of the blend.

[0064] In addition the respective behavior of compositions 1 and 3 werecompared between 80 and 160° C. For these two compositions, the strengthin N/1 5 mm for various sealing temperatures (ST) comprised between 80and 160° C. are given in the table below: ST in ° C. 80 90 100 110 120130 140 150 Composition 0.60 4.50 14.16 16.00 18.00 19.00 25.00 30.00 1Composition 1.36 2.30 2.73 3.36 3.32 3.60 4.26 4.90 3

[0065] These results show that the incorporation of Topas® 8007 in acomposition of Surlyn® 9910 (composition 3) reduces heat seal strengthand leads to a very constant low heat seal strength (between 1.36 and4.90) over a wide temperature field of said composition. On thecontrary, the heat seal strength of a composition without Topas® 8007(composition 1) constantly increases (from 0.60 to 30.00). The level ofheat seal strength can be adjusted by the amount of cycloolefin polymeradded to a polar derivative of an ethylene based polymer. This featureis characteristic for a highly performing peel blend for easy openingapplications.

[0066] Hot Tack Strength

[0067] Hot tack performance was measured with a commercial Packforsk hottack testing device in a temperature range between 80 and 110° C. usinga dwell time of 0.5 sec and a delay time of 0.1 sec at a pressure of 0.5MPa. Hot tack was measured on monolayer films.

[0068] Compositions 1, 3, 4, 5, 6, 7 and 8 were tested. The results,under the form of the strength in N/15 mm for various sealingtemperatures (ST) comprised between 80 and 110° C., are collected in thetable below: ST in ° C. 80 90 100 110 Composition 1 0.20 0.65 2.22 2.80Composition 3 0.98 1.46 3.93 4.20 Composition 4 0.56 1.70 5.10 4.20Composition 5 0.40 0.50 2.30 2.40 Composition 6 1.23 1.98 6.00 4.33Composition 7 0.70 3.03 1.36 — Composition 8 1.30 3.60 2.46 —

[0069] Compositions 3 and 4 are to be compared to composition 1.Composition 6 is to be compared to composition 5. Composition 8 is to becompared to composition 7. Hot Tack strength of polar derivatives ofethylene based polymers is greatly increased through addition ofcycloolefin polymers, despite the incompatibility of the two polymers.Moreover, although cycloolefin polymers have a melting point above 150°C. and polar derivatives of ethylene based polymers of the inventionhave a melting point of 85-95° C., the addition of 20% cycloolefinpolymers to these polar derivatives of ethylene based polymers reducesthe minimum temperature where hot tack strength is significantly high,therefore allowing automatic processing of the films made of the blendof the invention.

1. A blend comprising at least a polar derivative of an ethylene basedpolymer and at least a cycloolefin polymer.
 2. The blend of claim 1,wherein the ethylene based polymer comprises at least 60 weight % ofethylene, relative to the weight of the polymer and polar monomersselected from the group of carboxylic acids, alkyl acrylates and vinylacetates.
 3. The blend of claim 1, wherein the ethylene based polymer is0.01 to 99.5% neutralized with metal ions selected from sodium,potassium, zinc, calcium, magnesium, lithium, aluminum, nickel, andchromium.
 4. The blend of claim 1, wherein the polar derivative of theethylene base polymer is present in an amount from 50 weight % to 99.9weight %, relative to the weight of the blend.
 5. The blend of claim 1,wherein the cycloolefin polymer is built up from cycloolefins andcomonomers, where the comonomer content is at most 20% by weight, basedon the weight of the cycloolefin polymer.
 6. The blend of claim 5,wherein the cycloolefin polymer is an ethylene-norbornene copolymer. 7.The blend of claim 1, wherein the cycloolefin polymer is present in anamount from about 0.1 weight % to 50%, relative to the weight of theblend.
 8. A film comprising the blend of claim
 1. 9. A sealant layercomprising the film of claim
 8. 10. A packaging comprising the sealantlayer of claim
 9. 11. A method for improving the perforation resistanceof a film made of a polar derivative of an ethylene based polymercomprising the step of introducing therein from about 0.1 to 50% byweight, relative of the weight of the film, of a cycloolefin polymer.12. A method for reducing the heat seal initiation temperature of a filmmade of a polar derivative of an ethylene based polymer comprising thestep of introducing therein from about 0.1 to 50% by weight, relative ofthe weight of the film, of a cycloolefin polymer.
 13. A method forstabilizing the heat seal strength of a film made of a polar derivativeof an ethylene based polymer over a wide range temperature comprisingthe step of introducing therein from about 0.1 to 50% by weight,relative of the weight of the film, of a cycloolefin polymer.
 14. Amethod for increasing the hot tack strength of a film made of a polarderivative of an ethylene based polymer comprising the step ofintroducing therein from about 0.1 to 50% by weight, relative of theweight of the film, of a cycloolefin polymer.